1. Field of the Invention
The present invention relates to an anode in which a coat is provided on an active material layer, a method of manufacturing the same, a secondary battery including the anode, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), digital still cameras, mobile phones, Personal Digital Assistants, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery provides a higher energy density than that of a lead battery and a nickel cadmium battery. As an active material of the anode (anode active material) of the lithium ion secondary battery, a carbon material has been widely used.
In recent years, as high performance of the portable electronic devices is developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use tin, silicon or the like instead of the carbon material as an anode active material (for example, refer to U.S. Pat. No. 4,950,566). Since the theoretical capacity of tin (994 mAh/g) and the theoretical capacity of silicon (4199 mAh/g) are significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
However, in the lithium ion secondary battery with a high capacity, there has been the following disadvantage. The anode active material inserting lithium in charge becomes highly activated, and thus the electrolytic solution is easily decomposed and lithium is easily inactivated. As a result, when charge and discharge are repeated, the charge and discharge efficiency is lowered and thus it is not able to obtain sufficient cycle characteristics.
Accordingly, to improve the battery characteristics such as the cycle characteristics, an electrolytic solution containing a fluoro complex salt such as an alkali metal salt of hexafluoroantimonate, an alkali metal salt of hexafluorotantalate, lithium hexafluoroniobate, lithium hexafluorovanadate, lithium tetrafluoroferrate, lithium pentafluorotitanate, and lithium pentafluorozirconate; or a halogeno complex salt such as alkali metal periodate is used (for example, refer to Japanese Unexamined Patent Application Publication Nos. 58-204478, 63-310568, 03-152879, 06-290808, 2002-047255, and 2003-142154).
Further, to improve the cycle characteristics, forming an inert coat on the surface of the anode active material using a lithium compound such as lithium fluoride and lithium carbonate or an oxide such as silicon oxide and an aluminum oxide has been considered (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2004-327211, 07-302617, 10-255800, 11-135153, 2005-026230, 2005-142156, 2005-166469, and 2006-185728).